Usually, an outboard engine is adapted to: be fitted to a hull; have an exhaust outlet of an exhaust path (exhaust system) to discharge exhaust gas of its engine, submerged in the water that is the exterior of the hull; and discharge the exhaust gas in the exhaust path from the exhaust outlet into the water by rotating a propeller. An exhaust noise of the exhaust gas can be suppressed by discharging the exhaust gas into the water.
During the rotation of the engine in idling of the outboard engine (at the no-load minimum engine speed), the propeller for propulsion is maintained in its stoppage state and the exhaust pressure of the exhaust gas is low. Therefore, it is difficult to externally discharge the exhaust gas from the exhaust outlet through the seawater.
Therefore, an idling exhaust path is provided for the exhaust path at a halfway point of the exhaust path and the idling exhaust path is open to the atmosphere. Thereby, the exhaust gas produced during the rotation in idling can externally be discharged from the idling exhaust path. In this case, to suppress the exhaust noise of the exhaust gas discharged from the idling exhaust path, the idling exhaust path is provided with an idling chamber.
As to the outboard engine, by providing the idling exhaust path for the exhaust path at the halfway point of the exhaust path, occurrence of any negative pressure can be prevented in the exhaust path (exhaust system), for example, when the engine is started up or when the exhaust pulsates. An outboard engine is known as is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2007-283857, that can suppress the sucking up of seawater or freshwater from the exhaust path due to the negative pressure by preventing the occurrence of the negative pressure in the exhaust path as above.
However, the outboard engine disclosed in the above '857 publication also externally discharges the exhaust gas from the idling exhaust path when, for example, the engine drives with its high speed rotations. When the engine rotates at a high speed, the amount of the exhaust gas discharged is also increased. Therefore, the idling chamber provided for the idling exhaust path is overheated by the exhaust gas and, from this viewpoint, room for improvement of the outboard engine is still left.